Rc oscillator

ABSTRACT

A method includes using a current source to provide a charging current to a capacitor of a resistor-capacitor (RC) tank of an RC oscillator. The method includes using a resistor of the current source as a resistor for the RC tank.

BACKGROUND

Clock signals may be used in an electronic system for such purposes assynchronizing operations of a processor, keeping track of time andinitiating periodic activity. The clock signal is generated by anoscillator, and the frequency of the clock signal typically is regulatedby controlling the frequency of the oscillator. One way to control theoscillator's frequency is through the use of a resistor-capacitor (RC)tank, or resonant circuit, which may be part of the same integratedcircuit as the oscillator.

SUMMARY

In an example embodiment, a method includes using a current source toprovide a charging current to a capacitor of a resistor-capacitor (RC)tank of an RC oscillator. The method includes using a resistor of thecurrent source as a resistor for the RC tank.

In another example embodiment, an apparatus includes aresistor-capacitor (RC) tank circuit, a discharge path and a chargepath. The RC tank is formed from a resistor and a capacitor and has anoscillation cycle. In a first part of the cycle, the capacitor ischarged, and during a second part of the cycle the capacitor, thedischarge path circuit discharges the capacitor. The charge path circuitcommunicates a charging current to the capacitor. The charge pathcircuit includes an amplifier to amplify a voltage of the capacitor togenerate a signal to control coupling of the discharge path circuit tothe capacitor.

In yet another example embodiment, an apparatus includes an integratedcircuit, which includes a processor core and a clock system. The clocksystem includes clock sources, and the clock system is adapted to selecta clock signal provided by a given clock source and provide the selectedclock signal to at least one component of the integrated circuit. Thegiven clock source includes a resistor-capacitor (RC) oscillator, whichincludes a current source and an RC tank circuit. The current sourceprovides a regulated current and includes a resistor. The RC tankcircuit includes a capacitor and also includes the resistor of thecurrent source to establish a period of the clock signal.

Advantages and other desired features will become apparent from thefollowing drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electronic system according to anexample embodiment.

FIG. 2 is a schematic diagram of a microcontroller unit (MCU) of theelectronic system of FIG. 1 according to an example embodiment.

FIG. 3 is a schematic diagram of a resistor-capacitor (RC) oscillatoraccording to an example embodiment.

FIG. 4 is a waveform of a voltage generated by a capacitor charge pathcircuit of the oscillator of FIG. 3 according to an example embodiment.

FIG. 5 is a flow diagram depicting a technique to use a resistor of acurrent source as the resistor for an RC tank of an RC oscillatoraccording to an example embodiment.

FIG. 6 is a flow diagram depicting a technique to generate a signal tocontrol coupling of a discharge path circuit to a capacitor of an RCtank of an RC oscillator according to an example embodiment.

FIG. 7 is a more detailed schematic diagram of the RC oscillator of FIG.3 according to an example embodiment.

FIG. 8 is a schematic diagram of a current source according to a furtherexample embodiment.

FIG. 9 is a schematic diagram of an RC oscillator according to a furtherexample embodiment.

DETAILED DESCRIPTION

An electronic system, such as a microcontroller unit (MCU)-basedplatform, may be constructed to perform activities using a relativelyhigh frequency clock signal (a clock signal having a frequency in theMegaHertz (MHz) range, for example) and a relatively low frequency clocksignal (a clock signal having a frequency in the kiloHertz (kHz) range,for example). For an MCU-based electronic system, the low frequencyclock signal may be used for such purposes as supporting generation of areal time clock (RTC), providing a periodic wake up for the MCU toperform calibrations, allowing components of the system to check sensorinputs, or allowing other periodic activities.

Because the low frequency clock signal may be provided during a time oflow power consumption for the electronic system, an oscillator providingthe clock signal may also be constrained to have a relatively smallpower demand and thus, operate while consuming a relatively smalloverall current (a current of a few nanoamperes (nA), for example) whilemaintaining a reasonable accuracy (an accuracy around five percent, forexample).

In accordance with example embodiments, the electronic system uses aresistor-capacitor (RC) oscillator, a relatively low power and accurateoscillator, to provide a low frequency clock signal. The RC oscillatorhas an RC tank (which may also be referred to as a “resonant circuit,”“tank circuit,” and so forth) that establishes the resonance frequencyfor the oscillator. The RC tank includes a resistor and a capacitor; andthe oscillation frequency of the oscillator is proportional to theproduct of the resistor's resistance and the capacitor's capacitance.

For each cycle of oscillation, the RC oscillator charges and thendischarges the capacitor of the RC tank. In this manner, the RCoscillator has a charge path circuit to provide a charging current tothe capacitor to store energy in the capacitor during a charging part ofthe cycle, and the RC oscillator has a discharge path circuit to removethe stored energy from the capacitor during a discharging part of thecycle. The voltage of the capacitor rises during charging, and thecapacitor is discharged in response to the voltage of the capacitorreaching a voltage that is related to (equal to, for example) thevoltage that is generated by a current in a resistor of the RCoscillator.

In accordance with example embodiments, a current source of the RCoscillator provides the charging current that charges the capacitor ofthe RC tank, and a resistor of the current source serves dual functions:establishing the current that is provided by the current source; andfunctioning as the resistor of the RC tank. Moreover, in accordance withexample embodiments disclosed herein, the charging path circuit of theRC oscillator contains a single-ended amplifier that is constructed toprovide a first stage of amplification for a signal that controlscoupling of the discharge path circuit to the capacitor at the end ofthe charging part of the oscillation cycle. In accordance with someembodiments, the amplifier of the charging path circuit may provide mostof the amplification for the signal that controls thecoupling/decoupling of the discharge path circuit.

Referring to FIG. 1, as a more specific example embodiment, anelectronic system 100 includes a clock system 98 that contains multipleclock sources, including an RC oscillator 200. In particular, the clocksystem 98 selects and provides clock signals that may be used for anumber of purposes, as further described herein.

The electronic system 100, in accordance with example embodiments,includes a microcontroller unit (MCU) 24, which controls various aspectsof one or more components 70 of the electronic system 100. In general,the MCU 24 communicates with the components 70 via communicationinput/output (I/O) signals 74, which may be wireless signals; hardwiredcables-based signals; and so forth, depending on the particularembodiment. As examples, the components 70 may include such componentsas a lighting element; an electrical motor; a household appliance; aninventory control terminal; a computer; a tablet; a smart power meter; awireless interface; a cellular interface; an interactive touch screenuser interface; and so forth.

As depicted in FIG. 1, in accordance with example embodiments, all orpart of the components of the MCU 24 may be part of a semiconductorpackage 110. In this manner, all or part of the components of the MCU 24may be fabricated on a single die or on multiple dies, depending on theparticular embodiment, and encapsulated to form the semiconductorpackage 110.

Referring to FIG. 2 in conjunction with FIG. 1, in accordance withexample embodiments, the MCU 24 contains a processor core 150, digitalcomponents 90 and an analog system 96. As an example, the processor core150 may be a 32-bit core, such as an Advanced RISC Machine (ARM)processor core, which executes a Reduced Instruction Set Computer (RISC)instruction set. In further example embodiments, the processor core 150may be a less powerful core, such as an 8-bit core (an 8051 core, forexample). The digital components 90 may be, as examples, a UniversalSerial Bus (USB) interface; a universal asynchronousreceiver/transmitter (UART) interface; a system management bus interface(SMB) interface; a serial peripheral interface (SPI) interface; timers;and so forth. In general, the digital components 90 may communicate withdevices that are external to the MCU 24 via associated I/O signals 74-1.

The analog system 96 may include various analog components and systemsthat receive analog signals, such as analog-to-digital converters (ADCs)and comparators; as well as analog components that provide analogsignals, such as current drivers. In general, the analog system 96communicates with devices that are external to the MCU 24 via associatedI/O signals 74-2.

Among its other components, the MCU 24 includes a system bus 130 that iscoupled to the digital components 90, analog system 96 and processorcore 150. A memory system 158 is also coupled to the system bus 130. Thememory system includes a memory controller, or manager 160, whichcontrols access to various memory components of the MCU 24, such as acache 172, a non-volatile memory 168 (a Flash memory, for example) and avolatile memory 164 (a static random access memory (SRAM), for example).In accordance with example embodiments, the volatile memory 164 and thenon-volatile memory 168 may form the system memory of the MCU 24. Inother words, the volatile memory 164 and the non-volatile memory 168have memory locations that are part of the system memory address spacefor the MCU 24.

It is noted that FIG. 2 depicts a general simplified representation ofan example MCU architecture, as the MCU 24 have many other components,bridges, buses, and so forth, in accordance with further embodiments,which are not depicted in FIG. 2. For example, in accordance withfurther example embodiments, the MCU 24 may have a bus matrix modulethat responds to slave side arbitration to regulate access to the memorydevices of the MCU 24. Thus, many other embodiments are contemplated,which are within the scope of the appended claims.

The clock system 98 provides one or multiple clock signals at itsoutput(s) 221, which may be used in the MCU 24 for any of a number ofpurposes, depending on the particular embodiment. As depicted in FIG. 2,in some embodiments, the clock system 98 may provide a clock signal to areal time clock (RTC) module 225 of the MCU 24. The RTC module 225, inaccordance with example embodiments, is essentially a counter thatstores data in a way that allows components of the MCU 24 to request atime or a date. The RTC module 225 may include a scheduler to start agiven activity or send an interrupt to the processor core 150 when aparticular time is reached.

In further embodiments, a clock signal that is provided by the clocksystem 98 may be used to run a dedicated digital engine, such that ashort cycle of activity (10 to twenty clock cycles, for example) by thedigital engine begins in response to the rising edges of the clocksignal. In accordance with example embodiments, the clock system 98 mayalternate the frequency of a clock signal between relatively high (MHzfrequencies, for example) and low (kHz frequencies, for example)frequencies to initiate analog and digital cycles of activity. In thismanner, the clock system 98 may regulate provide a relatively lowfrequency clock signal for several cycles to change analog states,subsequently provide a relatively high frequency clock signal forseveral cycles initiate digital activity and then the circuitry enters awaiting state for the next low frequency rising clock edge.

In further embodiments, a the clock system 98 may provide a relativelylow frequency clock signal that is used to clock a digital watchdogtimer of the MCU 24 such that if no activity occurs for a certain numberof low frequency clock cycles, the MCU 24 takes a predetermined actiondue to the inactivity. For example, after a certain number of lowfrequency cycles of inactivity, the MCU 24 may power down components forpurposing of reducing power consumption. As another example, the MCU 24may reset one or more components after a certain number of low frequencycycles of inactivity, as the inactivity may indicate a hung state.

For a relatively low frequency clock signal, the clock system 98 mayselect a clock signal (called “CLK” in FIG. 2), which is provided by theRC oscillator 200. The clock system 98 may have one or multiple otherclock sources 210, which generally provide different frequency clocksignals for the MCU 24 and which may be selected by the clock system 98for purposes of providing the system's clock signal(s). FIG. 2schematically depicts selection of the clock sources via a multiplexer220 that may be controlled (via selection signals 224 provided theprocessor core 150, for example) for purposes of selecting theappropriate frequency clock signal(s) that are provided at the output(s)221 of the clock system 98. Referring to FIG. 3, in accordance withexample embodiments, the RC oscillator 200 includes a current source300; and an RC tank, which is formed from a capacitor 310 and a resistor304. As depicted in FIG. 3, in accordance with example embodiments, theresistor 304 of the RC tank is part of the current source 300.

The RC oscillator 200 charges and discharges the capacitor 310 in eachoscillation cycle of the oscillator 200 to correspondingly form onecycle of the CLK clock signal. The charging of the capacitor 310 occursin a charging part of the oscillation cycle. In this part of the cycle,the current source 300 provides a relatively fixed, or constant, current(called “I₂” in FIG. 3), which produces a charging current (called “I₃”)in a charge path circuit (called a “charge path 330” herein) for thecapacitor 310. In accordance with some embodiments, the magnitude of theI₃ charging current is generally proportional to (equal to, for example)the magnitude of the I₂ current. In the charging part of the cycle, thecapacitor 310 charges in response to the I₃ charging current, and avoltage of the capacitor 310 (called “V_(cAP)” in FIG. 3) correspondingramps upwardly.

In accordance with example embodiments, the charge path 330 contains asingle-ended amplifier 314 that amplifies the V_(CAP) capacitor voltage(as described further herein) to provide a voltage (called “V₁” in FIG.3) that is used to control the coupling and decoupling of a dischargepath circuit (called a “discharge path 331 in FIG. 3) to/from thecapacitor 310; and as such, V₁ voltage serves as a signal to controlwhen the charging and discharging parts of the oscillation cycle beginand end.

For the example embodiment that is depicted in FIG. 3, the dischargepath 331 is form by a controlled current path, or switch 320. In thismanner, for the charging part of the cycle, the switch 320 does notconduct, or is turned off; and during the charging cycle, the V₁ voltagegenerally increases in magnitude over time as the capacitor 310 is beingcharged and the V_(CAP) capacitor voltage rises. As described furtherbelow, due to the charging of the capacitor 310, the V₁ voltageeventually reaches a voltage, which causes the switch 320 to conduct, orturn on, and consequently begin the discharging part of the cycle inwhich the capacitor 310 is discharged through the switch to ground. Thedischarging of the capacitor 310, in turn, causes the V₁ and V_(CAP)voltages to decrease; and the lowered V₁ voltage causes the switch 320to turn off, thereby ending the charging part of the current cycle andbeginning the charging part of the next cycle.

The magnitude of the I₂ current is generally inversely proportional tothe resistance of the current source's resistor 304, in accordance withexample embodiments. In this manner, as depicted in FIG. 3, the currentsource 300 produces a current (called “I₁” in FIG. 3) in the resistor304, which is proportional in magnitude to the magnitudes of the I₂ andI₃ currents. The I₁ current produces a voltage (called “V_(R)” in FIG.3) across the resistor 304. The V_(CAP) capacitor voltage charges at atime rate that is proportional to the I₂ current divided by thecapacitance of the capacitor 310. Because the I₃ and I₁ currents areproportional, the cycle time, in accordance with example embodiments, isproportional to the product of the resistance of the resistor 304 andthe capacitance of the capacitor 310 and is independent of the I₁, I₂and I₃ currents, and when the V_(CAP) capacitor voltage reaches athreshold voltage that is proportional to the resistance of the resistor304 and the I₁ current, the charging of the capacitor 310 ends and thedischarging of the capacitor 310 begins. The cycle time is the thresholdvoltage divided by the time rate, in accordance with exampleembodiments. The charging part of the oscillation cycle is proportionalto the product of the resistance of the resistor 304 and the capacitanceof the capacitor 310. In accordance with example embodiments, thedischarging part of the cycle is a set by a propagation delay time(described below); and the product of the resistance of the resistor 304and the capacitance of the capacitor 310 establishes the periodicfrequency of the oscillation cycle.

In accordance with example embodiments, the RC oscillator 200 includes adigital buffer 324 that pulse shapes the V₁ voltage (i.e., sharpens therising and falling edges of the V₁ voltage) to provide the CLK signal atthe output of the buffer 324. It is noted that depending on theparticular embodiment, the oscillator 200 may contain frequency dividersand other circuitry to change the frequency of the CLK signal, changethe duty cycle of the CLK signal, and so forth. As described furtherherein, in accordance with example embodiments, the propagation delay ofthe digital buffer 324 sets the duration of the discharge part of theoscillation cycle.

Thus, referring to FIG. 5, in accordance with example embodiments, atechnique 500 includes using (block 504) a current source to provide acharging current to a capacitor of a resistor-capacitor (RC) tank of anRC oscillator. Pursuant to the technique 500, a resistor of the currentsource is used (block 508) as a resistor for the RC tank.

A potential advantage of using a resistor of a current source as part ofthe RC tank is that die area is conserved (i.e., one less resistor isused). Another potential advantage is that current (and therefore powerconsumption) may be reduced due to the use of part of the current sourcefor dual functions.

Referring back to FIG. 3, in accordance with some embodiments, aparticular advantage of having the single-ended amplifier 314 disposedin the charge path 330 is that the amplifier 314 may be used in place ofan input stage of a comparator that may otherwise be used to amplify theV_(CAP) capacitor voltage. In this manner, the amplifier 314, inaccordance with example embodiments, provides most of the amplificationfor the CLK signal that controls the coupling of the discharge pathcircuit 331 to the capacitor 310. FIG. 4 depicts an example magnitudeversus time waveform of the V₁ voltage for an example oscillation cyclefrom time T₀ to time T₃. Charging of the capacitor 310 occurs from timeT₀ to time T₂; and discharging of the capacitor 310 occurs from time T₂to time T₂.

From time T₀ (when the capacitor 310 discharged) to time T₁, themagnitude of the V₁ voltage generally linearly ramps upwardly andgenerally follows the corresponding ramping of the V_(CAP) capacitorvoltage. Near or at time T₁, the magnitude of the V₁ voltage reaches athreshold voltage magnitude (called “V₂” in FIG. 4), which is set by theresistance of the resistor 304.

As further described herein, when the magnitude of the V₁ voltagereaches the V₂ threshold at time T₁ the amplification that is applied byamplifier 314 of the charge path 330 increases. While the V_(CAP)capacitor voltage is lower than the V₂ voltage, the V₁ voltage isslightly higher than the V_(CAP) capacitor voltage. As the V_(CAP)capacitor voltage approaches and exceeds the V₂ voltage, the V₁ voltageincreases rapidly from time T₁ to time T₂, as depicted in FIG. 4, whichcauses the voltage at the control terminal of switch 320 to a largermagnitude (called “V₃” in FIG. 4) turn on the switch 320 and triggerdischarging of the capacitor 310. The discharging the capacitor 310occurs from time T₂ to time T₃. At time T₃, the capacitor 3120 isdischarged and the switch 320 opens to begin another oscillation cycle.

Thus, referring to FIG. 6, in accordance with example embodiments, atechnique 600 includes providing (block 604) a current to a charge pathcircuit of an RC oscillator to charge a capacitor of the oscillator's RCtank during the charging part of the oscillator's cycle. The technique600 further includes amplifying a voltage of the capacitor to generate asignal to control coupling of a discharge path circuit of the RCoscillator to the capacitor.

A potential advantage in using an amplifier in the charge path is thatdie area, otherwise consumed by a comparator, for example, may beconserved; and another potential advantage is that power may beconserved due to reduction in circuitry.

Referring to FIG. 7, in accordance with example embodiments, the currentsource 300 is a V_(GS)/R current source circuit, which is a self-biasedcurrent source. The current source 300 includes an N-channelmetal-oxide-semiconductor field-effect-transistor (NMOSFET) 710, whichhas its gate and source coupled across the resistor 304. Thus, thevoltage across the resistor 304 is equal to the gate-to-source (V_(GS))of the NMOSFET 710; and the magnitude of the I₁ current is equal to thequotient of the V_(GS) voltage of the NMOSFET 710 divided by theresistance of the resistance 204. For this example embodiment, thesource of the NMOSFET 710 is coupled to ground; one terminal of theresistor 304 is coupled to ground; and the other terminal of theresistor 304 is coupled to the gate of the NMOSFET 710. An NMOSFET 720of the current source 300 has its source coupled to the gate of theNMOSFET 710, and a gate of the NMOSFET 720 is coupled the drain of theNMOSFET 710.

A current mirror of the current source 300, which is formed fromP-channel MOSFETs (PMOSFETs) 714 and 718, mirrors the I₁ current intothe drain-to-source path of the NMOSFET 710 (i.e., into thecurrent-controlled path of the NMOSFET 710). In this regard, the sourceof the PMOSFET 718 is coupled to the V_(DD) supply voltage, and thedrain of the PMOSFET 718 is coupled to the drain of the NMOSFET 720.Moreover, the gate and drain of the PMOSFET 718 are coupled together.The source of the PMOSFET 714 is coupled to the V_(DD) supply voltage, agate of the PSMOFET 714 is coupled to the gate and drain of the PMOSFET718; and the drain of the PMOSFET 714 is coupled to the drain of theNMOSFET 710.

In accordance with example embodiments, the I₁ current has a magnitudeequal to the gate-to-source voltage (V_(GS)) of the NMOSFET 710 dividedby the resistance (R) of the resistor 304, or V_(GS)/R, which may beapproximated as “V_(t)/R” (where “V_(t)” represents the MOSFET thresholdvoltage of the NMOSFET 710).

As depicted in FIG. 7, the current source 300 further includes a PMOSFET724 that additionally mirrors the I₁ current to produce the I₂ currentin the charge path 330. The I₂ current is proportional to the I₁ and maybe equal to, a multiple of, or a fraction of the I₁ current, dependingon the relative aspect ratios of the PMOSFETs 718 and 724. The source ofthe PMOSFET 724 is coupled to the V_(DD) supply voltage; the gate of thePMOSFET 724 is coupled to the gates of the PMOSFETs 714 and 718; and thedrain of the PMOSFET 724 provides the I₂ current.

The amplifier 314 of the charge path 330 includes an NMOSFET 730, inaccordance with example embodiments. In this regard, the I₂ currentpasses through the drain-to-source path of the NMOSFET 730 and, for thisexample, is near or equal to the I₃ current, as negligible current isassumed to be communicated to the digital buffer 324. The drain of theNMOSFET 730 is coupled to the drain of the PMOSFET 724; the gate of theNMOSFET 730 is coupled to the gate of the NMOSFET 720; and the source ofthe NMOSFET 730 is coupled to the input of the current buffer 324 andthe non-ground terminal of the capacitor 310.

Also for the example embodiment depicted in FIG. 7, the switch 320includes an NMOSFET 740, whose drain-to-source path is coupled acrossthe capacitor 310. The gate of the NMOSFET 740 is coupled to the CLKclock signal, i.e., coupled to the output of the digital buffer 324.

The digital buffer 324 may take on numerous forms, depending on theparticular embodiment. In an example embodiment, the digital buffer 324is formed from an even number of serially-coupled complementarymetal-oxide-semiconductor (CMOS) inverters (i.e., a CMOS inverterchain).

The RC oscillator 200 operates as follows. In general, the NMOSFETs 720and 730 are considered to be matching transistors for this exampleembodiment. The effective charging reference voltage is determined bythe V_(R) resistor voltage. As long as the V_(CAP) capacitor voltage isless than the V_(R) resistor voltage, the currents through thedrain-to-source paths of the NMOSFETs 720 and 730 are equal or fixedratios of each other, depending on the aspect ratios of the PMOSFETs 718and 724.

When the capacitor 310 is being charged, the NMOSFET 740 does notconduct, or is “turned off,” and the V_(CAP) capacitor voltage has arelatively low magnitude. Due to the charging of the capacitor 310, theV_(CAP) capacitor voltage ramps upwardly. The NMOSFET 730 is operatingin a linear mode of operation during the initial charging of thecapacitor 310, which causes the magnitude of the V₁ voltage to generallyfollow the magnitude of the V_(CAP) voltage. When the V_(CAP) capacitorvoltage reaches the V_(R) resistor voltage, the magnitudes I₃ currentdecreases abruptly, due to the gates of the NMOSFETs 720 and 730 beingcoupled together and the NMOSFET 730 turning off. The PMOSFET 724 pullsthe V₁ voltage to or near the V_(DD) supply voltage, the I₂ currentdecreases and the NMOSFET 740 turns on. Thus, at this point, the chargepath 330 is decoupled from the capacitor 310, and the discharge path 331is coupled to the capacitor 310.

In accordance with example embodiments, the digital buffer 324introduces a propagation delay that sets the duration of the dischargingpart of the oscillation cycle. In this manner, the turning on of thePMOSFET 724 causes the V₁ voltage to be pulled toward ground. However,the appearance of the lower V₁ magnitude at the gate of the NMOSFET 740is delayed by the propagation delay of the digital buffer 324. Thepropagation delay of the digital buffer 324 (or other introduced delay,depending on the particular embodiment) is selected to ensure that thecapacitor 310 is fully discharged before the charging part of the cyclebegins. As an example, in accordance with some embodiments, thepropagation delay is a function of the number of CMOS inverters that areused to form the digital buffer 324.

Other embodiments are contemplated, which are within the scope of theappended claims. In this manner, FIG. 8 depicts a current source 800 inaccordance with a further example embodiment. The current source 800 isa delta V_(GS)/R current source circuit. Elements of the current source800 shared in common with the current source 300 are denoted usingsimilar reference numerals. The current source 800 has NMOSFETs 810 and814 that replace the NMOSFET 710 of the current source 300. In thisregard, the NMOSFET 810 has its source coupled to ground and its drainand gate coupled together. Moreover, the drain of the NMOSFET 810 iscoupled to the drain of the PMOSFET 714. The gate of the NMOSFET 814 iscoupled to the gate of the NMOSFET 810, the source of the NMOSFET 814 iscoupled to node 305, and the drain of the NMOSFET 814 is coupled to thegate and drain of the PMOSFET 718. Moreover, the gates of the NMOSFETs810 and 814 are coupled (as indicated by terminal 820) to the NMOSFET730 (see FIG. 7).

In accordance with example embodiments, the I₂ current has a magnitudethat generally proportional to “(V_(Gs1)−V_(GS2))/R,” where “V_(GS1)”and “V_(GS2)” represent the gate-to-source voltages of the NMOSFETs 810and 814, respectively; and “R” represents the resistance of the resistor304. The V_(GS1)−V_(GS2) voltage is largely determined by the relativeaspect ratios of the PMOSFETs 718 and 714 and the relative aspect ratiosof NMOSFETs 814 and 810, which also set the relative current densitiesin the NMOSFETs 810 and 814. In some embodiments 810 and 814 could bechosen have different threshold voltages, and the I₂ current may beapproximated as “(V_(t1)−V_(t2))/R,” where “V_(t1)” and “V_(t2)”represent the MOSFET threshold voltages of the NMOSFETs 810 and 814,respectively.

The RC oscillator may use other current sources, in accordance withfurther embodiments.

As another example of a further embodiment, FIG. 9 depicts an RCoscillator 900 that does not include a current source, but rather, theresistor 304 of the RC tank receives a reference current. In general,the RC oscillator 900 has features similar to other RC oscillatorsdiscussed herein, with the same reference numerals being used to denotesimilar components and different reference numerals being used to denotethe new features. Unlike the oscillator 200, for the oscillator 900, aPMOSFET 904 mirrors a current from a current source (not depicted inFIG. 9), and this mirrored current is routed through the resistor 304,which is coupled in series with the source-to-drain path of the PMOSFET904. Moreover, an NMOSFET 906 has its drain-to-source path coupled inseries with the source-to-drain path of the PMOSFET 904 and the resistor304. As shown in FIG. 9, the drain and gate of the NMOSFET 906 arecoupled together; and the gate of the NMOSFET 906 is coupled to the gateof the NMOSFET 730. The resistor 304 is coupled between the source ofthe NMOSFET 906 and ground.

As yet another example of a further example embodiment, the circuitrythat is described herein may be replaced by equivalent CMOS circuitry inwhich the PMOSFETs and NMOSFETS are interchanged, as can be appreciatedby one of ordinary skill in the art.

While a limited number of embodiments have been disclosed herein, thoseskilled in the art, having the benefit of this disclosure, willappreciate numerous modifications and variations therefrom. It isintended that the appended claims cover all such modifications andvariations.

1. A method comprising: using a current source to provide a chargingcurrent to a capacitor of a resistor-capacitor (RC) tank of an RCoscillator; using a resistor of the current source as a resistor for theRC tank; and communicating the charging current to the capacitor using acharging path, wherein using the charging path comprises communicatingthe charging current to the capacitor using an amplifier and using theamplifier to amplify the voltage to generate a signal to turn on adischarge path to discharge the capacitor.
 2. The method of claim 1,further comprising using a resistance of the resistor to regulate thecharging current.
 3. The method of claim 1, wherein using the resistorcomprises: providing a first transistor having a controlled current pathin series with the resistor; providing a second transistor having acontrolled current path in series with the capacitor; and couplingcontrol terminals of the first and second transistors together.
 4. Themethod of claim 3, wherein the coupling comprises: providing atransistor having a current control path in series with the resistor;and coupling a control terminal of the transistor to a node of thecurrent source.
 5. The method of claim 1, further comprising: using theRC oscillator to in a clock system for an electronic device to provide arelatively low frequency clock signal to change a state of an analogcomponent of the electronic device and using a clock source of the clocksystem other than the RC oscillator to provide a relatively highfrequency clock signal to change a state of a digital component of theelectronic device.
 6. (canceled)
 7. The method of claim 1, furthercomprising activating a transistor to discharge the capacitor.
 8. Themethod of claim 1, wherein using the current source comprises using aV_(GS)/R current source or a delta V_(GS)/R current source to providethe charging current.
 9. An apparatus comprising: a resistor-capacitor(RC) tank circuit formed from a resistor and a capacitor and having anoscillation cycle comprising a first part of the cycle in which thecapacitor is charged and a second part of the cycle in which thecapacitor is discharged; a discharge path circuit to discharge thecapacitor during the second part of the cycle; and a charge path circuitto communicate a charging current to the capacitor, wherein the chargepath circuit comprises an amplifier to communicate the charging currentand to amplify a voltage of the capacitor to generate a signal tocontrol coupling of the discharge path circuit to the capacitor.
 10. Theapparatus of claim 9, wherein the amplifier comprises a single-endedamplifier to amplify the voltage.
 11. The apparatus of claim 9, whereinthe amplifier comprises a transistor to operate in a linear mode ofoperation to communicate the charging current to the capacitor andtransition to an off state near or at the end of the first part of thecycle to increase the amplification of the signal.
 12. The apparatus ofclaim 9, wherein the discharge path circuit comprises a switch to beselectively turned on by the signal to discharge the capacitor.
 13. Theapparatus of claim 9, further comprising a digital buffer to reshape thevoltage of the capacitor to generate a clock signal.
 14. An apparatuscomprising: an integrated circuit comprising a processor core and aclock system comprising a plurality of clock sources and being adaptedto select a clock signal provided by a given clock source of theplurality of clock sources and to provide the selected clock signal toat least one component of the integrated circuit, wherein the givenclock source comprises a resistor-capacitor (RC) oscillator comprising:a current source to provide a regulated current and comprising aresistor; an RC tank circuit comprising the resistor and a capacitor toestablish a period of the clock signal; and a charge path circuit tocharge the capacitor using a charging current, wherein the charge pathcircuit comprises an amplifier to communicate the charging current tothe capacitor and amplify a voltage of the capacitor to generate asignal to control operation of a discharge circuit to discharge thecapacitor.
 15. The apparatus of claim 14, wherein a resistance of theresistor controls a magnitude of the charging current.
 16. The apparatusof claim 14, wherein the current source comprises: a first transistorhaving a first controlled current path in series with the resistor; asecond transistor having a second controlled current path in series withthe capacitor; and a current mirror to couple the first and secondcontrolled current paths together.
 17. The apparatus of claim 14,wherein the RC oscillator comprises a charge path circuit to charge thecapacitor, the charge path circuit comprising: a transistor having acurrent controlled path in series with the capacitor, wherein a controlterminal of the transistor is coupled to a node of the current source tocause the transistor to turn off in response to a voltage of thecapacitor reaching a threshold established by the resistor. 18-19.(canceled)
 20. The apparatus of 14, wherein the current source comprisesa V_(GS)/R current source or a delta V_(GS)/R current source.
 21. Theapparatus of 14, wherein the RC oscillator is associated with anoscillation cycle, and the amplifier comprises a transistor to operatein a linear mode of operation to communicate the charging current to thecapacitor and transition to an off state near or at the end of a firstpart of the oscillation cycle to increase the amplification of thesignal.
 22. The method of claim 1, wherein using the amplifier toamplify the voltage comprises operating a transistor in a linear mode ofoperation to communicate the charging current to the capacitor andtransitioning the transistor to an off state near or at the end of afirst part of an oscillation cycle for the RC oscillator to increase theamplification of the signal.